Multimedia communication terminals which can perform digital image communication have been popularized really. As a main factor for putting digital image communication into a practice, there can be enumerated a progress of image information compression technique (image coding technique) in addition to the development of electronic device technique. Image encoding technique is a technique for compressing image information having an information amount 1000 times as large as that of audio information to the level such that image information can be transmitted/stored. When a digital image communication using image encoding techniques is carried out, a terminal on the transmission side encodes (compresses), transforms inputted image information into an encoded bit stream, and transmits the encoded bit stream. A terminal on the reception side obtains a decoded image by decoding a received bit stream. At that time, in order to correctly carry out the communication, terminals located on both ends of the channel have to use a common image encoding system. To this end, as an international standard, there are now determined standard image encoding systems such as H. 261, MPEG1 or MPEG2. It is expected that much more standard image encoding systems will be determined from now on in accordance with new applications such as a wireless image communication.
Most of image communication terminals that were already commercially available on the market are wired image communication terminals connected to an ISDN network line or the like. However, it is expected that wireless image communication terminals which can provide similar functions will become widespread in the future wireless networks. The wireless communication terminal is featured in its portability, and it is customary that the wireless communication terminal uses a storage battery as its electric power source. However, an amount of an electric power that a light-weight storage battery used by the portable terminal can supply is limited so that a power consumption of a terminal has to be suppressed in order to prolong a continuous communication time. Also, since a surveillance system or the like uses a large number of terminals, even when a storage battery is not in use, it is important to suppress the power consumption. While the saving of the electric power becomes an important condition depending on applications as described above, an image encoding algorithm having a high information compression ratio has a large amount of calculations, and a power consumption tends to increase. In general, it is a first object for the image encoding to increase an information compression ratio, and a problem of a power consumption is not regarded as being important. However, in the future in which terminals for multimedia communication will become widespread and the problem of the power consumption that has been regarded as being unimportant will hold an important position in the design and development of apparatus.
In order to save an electric power of commercially-available portable apparatus, notebook-type (laptop) computers had created some devices. Main power-saving functions of Macintosh Power Book 18C manufactured by Apple Computer Inc., USA are as follows:
(a) When a battery remaining amount becomes lower than a constant amount, a backlight of a liquid-crystal display is automatically darkened; PA1 (b) When a battery remaining amount becomes lower than a certain constant level, an alarm message is displayed on a display; PA1 (c) If a laptop computer, which is being energized, is left inoperative during a predetermined period of time, then it is automatically placed in a sleep mode (state in which all functions except for holding the memory contents of RAM are stopped); PA1 (d) A user can switch a clock frequency of a CPU, and it becomes possible to save an electric power by lowering a processing speed; and PA1 (e) When a CPU is not requested in processing during a predetermined time, a clock frequency of the CPU is lowered automatically. Such a countermeasure for decreasing a power consumption in the laptop computer causes a display screen to become difficult to see and also causes a processing speed to be lowered. However, in general, the content of processing itself and provided information itself (e.g., information itself to be displayed on the display screen) are not changed.
In general, since an encoding algorithm having a high information compression ratio executes a complex processing, a processing amount (calculation amount) tends to increase. As the calculation amount increases, a power consumption also is caused to increase. For example, in a terminal for multimedia communication having a storage battery as an electric power source or a communication terminal in which a power consumption has to be decreased, a continuous communication time is as a result reduced.
Therefore, an object of the present invention is to provide a communication terminal in which a power consumption can be saved and a necessary continuous communication time can be maintained. In particular, it is an object of the present invention to provide a communication terminal in which a power consumption can be decreased by changing the encoding mode.